This invention relates to the art of manufacturing reinforced plastic pipe.
Fiber reinforced resin pipe and tubular articles are typically made by winding a band of reinforcing fiber rovings such as fiberglass on a rotating mandrel in a helical pattern with several superimposed layers. This is accomplished by drawing the rovings through a delivery head which runs to and fro along the length of the rotating mandrel. A high percentage of the glass reinforced plastic pipe used in the United States is produced by the wet winding process, where a band of rovings is drawn through liquid resin to impregnate and coat them with liquid resin before they are applied to the mandrel.
The production rate of reinforced plastic pipe in the wet winding process is limited by three factors. First, the rotating mandrel slings uncured resin. It is an inherent characteristic of the conventional wet winding process that the saturated band of rovings arrives at the mandrel with more resin than it can retain once it is on the mandrel. This excess resin is squeezed out to the surface of the band by the tension in the rovings immediately after the band contacts the mandrel. At high mandrel speeds the resin is slung. This tends to cause fouling of the equipment, danger to operating personnel and waste of valuable raw materials.
One means of preventing slinging is simply to rotate the mandrel at a speed well below the slinging limit and let the resin drip off the pipe during and immediately after the winding process as in U.S. Pat. No. 3,519,520. This drip method requires that the popular 2" and 3" pipe sizes be wound at low rotational speeds to avoid severe resin slinging. This last limitation directly reduces the productivity of the process. A further disadvantage of the drip method is that the drippings are normally collected in a long trough under the mandrel and eventually reused. Thus this method is obviously limited to use with resins with a very long pot life. In the case of a thermosetting epoxy resin, slung resin cannot be recycled to the process since it typically cures before it can be returned to the resin bath. Furthermore, the collected resin frequently must be reprocessed to replace lost volatiles, etc. before it can be reused. In addition, the long trough required tends to increase clean-up problems and is a fire hazard. Therefore, the drip method is unattractive, especially for the production of small diameter pipe.
Bradley discloses in U.S. Pat. No. 3,616,063 a pipe making machine with a rotating elongated roller provided with a continuous spiral thread throughout its entire length for compacting and arranging resin deposited upon the mandrel. According to Bradley's description, this apparatus only rearranges the resin, and does not remove excess resin applied to the mandrel. Therefore, Bradley's apparatus does not alleviate the resin slinging problem.
Another means of dealing with the excess resin is to remove it manually with a squeegee into a hand-held container during the winding process. The collected resin in this case can usually be returned directly to the resin bath for immediate reuse. However, this method is unsatisfactory because the squeegee has a solid blade which either removes too little or too much resin. If too little resin is removed, the resin slinging problem is not solved, heavy drippage during curing occurs, and the exterior of the cured pipe has an uneven surface. If too much resin is removed, there is an inadequate resin layer between layers of rovings. This can result in increase is void content of the pipe.
A high void content is undesirable because it increases the risk of damaging chemicals permeating the pipe wall structure or leaking completely through it. Thus, it is important to leave enough excess resin on the surface of rovings as they are wound to assure a dense structure. It is also desirable to leave a moderate excess of resin on the surface of the last layer of rovings to serve as a weather and scuff resistant coating for the finished pipe.
A second limitation on production rates is whipping and oscillation of the mandrel as the mandrel rotates at speeds approaching its critical speed. At moderate amplitudes these oscillations make accurate winding of the mandrel with the roving band impossible. At higher amplitudes these oscillations can damage the machine and can endanger the operator.
A third limitation on production rates is inability to impregnate rovings with resin at high speeds. For example, when impregnating fiberglass rovings with an epoxy resin, it has been found that at roving speeds above about 300 to 350 linear feet per minute, it is difficult to coat the rovings adequately with epoxy.
Which of these three factors actually limits the production rate of a given piece of pipe depends upon many variables, including the diameter and length of the mandrel, nature of the rovings, means of supporting the mandrel, temperature and viscosity of the resin, etc. Generally resin slinging and oscillations resulting from rotating the mandrel at speeds approaching the first critical speed limit production rates for pipe diameters up to about 5 inches.
The industry needs a method and apparatus for increasing the production rate of reinforced plastic pipe and which provide efficient recycling of excess resin.